With recent development of electronic industry, there is a demand for miniaturization and high reliability of electronic products and devices. To meet the demand, various attempts have been made to form circuit patterns or electrodes of electronic products requiring high integration. Under such circumstances, use of a conductive metal paste is the center of interest because it hardly generates by-products or contaminants during a process.
A typical metal paste includes a conductive metal, a glass frit and an organic binder. The conductive metal includes silver, aluminum or the like. Of them, silver is generally used. The conductive metal paste is mainly used to mount a hybrid IC or a semiconductor IC, or to form various condensers or electrodes, and recently has the expanded application range to high-tech electronic products such as PCBs, ELs, touch panels, RFIDs, LCDs, PDPs or solar cells. With expansion and development of the related industry, a demand for the conductive metal paste is increasing.
In particular, as it is expected that conventional energy sources such as oil or charcoal will be exhausted, interests in alternative energy are increasing. Among them, a solar cell has abundant energy sources and does not cause an environmental pollution, and thus it becomes an object of attention.
The solar cell is classified into a solar heat cell that produces vapor required to run a turbine using a solar heat, and a solar light cell that converts photons into an electrical energy using properties of a semiconductor. Generally, the solar light cell (hereinafter, referred to as a solar cell) is represented as a solar cell.
The solar cell largely includes a silicon solar cell, a compound semiconductor solar cell and a tandem solar cell according to the raw material. Among these three solar cells, the silicon solar cell leads the solar cell market.
FIG. 1 is a cross-sectional view illustrating a basic structure of a silicon solar cell. Referring to this figure, the silicon solar cell includes a substrate 101 of a p-type silicon semiconductor, and an emitter layer 102 of an n-type silicon semiconductor. A p-n junction is formed at an interface between the substrate 101 and the emitter layer 102 in the similar way to a diode. In addition, FIG. 2 is a schematic view of the constitution of a front electrode in the solar cell structure. As shown in FIG. 2, the front electrode of the solar cell includes Ag formed on a front side and conductive aluminum and silver formed on a back side of the substrate. In this regard, the front electrode is, not shown in the figure, connected to the emitter layer by penetrating through an anti-reflection film during formation of the silicon solar cell.
When light falls on the solar cell having the above-mentioned structure, electrons and electron holes are created in a silicon semiconductor doped with impurities by the photovoltaic effect. For reference, electrons are created in the emitter layer 102 of an n-type silicon semiconductor as a plurality of carriers, and electron holes are created in the substrate 101 of a p-type silicon semiconductor as a plurality of carriers. The electrons and electron holes created by the photovoltaic effect are drawn toward the n-type silicon semiconductor and the p-type silicon semiconductor, and move to the front electrode 103 on the emitter layer 102 and a back electrode 104 below the substrate 101, respectively. Then, these electrodes 103, 104 are connected by a wire, so that the current flows.
The conductive metal paste is used to form the front or back electrode in the solar cell, in addition to various electrodes of other electronic products as mentioned above.
The front electrode (Ag electrode) of a commercial crystalline silicon solar cell is formed by a screen printing process. The front electrode formed on the front side of the silicon substrate needs to have a smaller line width and a larger line height to improve electrical conversion efficiency.
However, it is difficult to achieve a small line width during sintering, after screen-printing of the conventional silver paste on the substrate. It is also difficult to form a stable contact between the front electrode and the emitter of the solar cell during the sintering process of the substrate, after the screen-printing process of the paste. Thus, a broad range of thermal treatments is not available.